The sorption of pollutants on adsorbents is receiving increased attention both from the view of removal and recovery of pollutants from gas mixtures, especially the pollutants produced through combustion processes. The generation and discharge of carbon dioxide into the atmosphere due to the consumption of large quantities of fossil fuels has emerged as a significant pollution problem for the environment. Thus, studies are in progress increasingly to address this issue.
Moreover, the removal of carbon dioxide is also important in several gas purification operations, such as, the production of hydrogen gas, landfill and natural gas treatment, and in the purification of hydrocarbons. Various separation techniques are applicable for the removal of carbon dioxide, such as, adsorption, absorption, and membrane separation.
Processes based on the selective adsorption of a gas mixture generally involve contacting the gas mixture with the selective adsorbent in an adsorption zone. The adsorption zone is maintained at adsorption conditions (i.e., temperature and/or pressure) favorable to selectively adsorbing a component of the gas mixture and producing an adsorption effluent, which has a reduced concentration of the adsorbed component relative to the gas mixture. Subsequently, the adsorbable component is then desorbed by changing the conditions in the adsorption zone to induce desorption. Alternatively, the selective adsorbent can be moved from the adsorption zone to a desorption zone having conditions favorable for desorption. Under desorption conditions the adsorbable component is purged from the selective adsorbent. Following the desorption step, the adsorption zone is purged to remove the adsorbed component.
In general, there are three types of adsorption/desorption processes, and these include pressure swing adsorption, thermal (or temperature) swing adsorption, and combinations thereof.
In pressure swing adsorption (PSA) processes the gas mixture is fed to at least one of a plurality of adsorption zones having an adsorbent at a reduced pressure effective to adsorb at least one component of the gas mixture. After a defined time, the feedstream to the gas mixture is terminated and the adsorption zone is depressurized. Alternatively, the adsorbent can be moved into a desorption zone. In either case, the pressure is elevated to a defined level, which permits the separated adsorbed component to be drawn off.
In thermal swing adsorption (TSA) processes, the gas mixture is fed to at least one of a plurality of adsorption zones having an adsorbent at a reduced temperature effective to adsorb at least one component of the gas mixture. After a defined time, the feedstream to the adsorbent is terminated and the temperature in the adsorption zone is increased. Alternatively, the adsorbent is moved to a desorption zone. In either case, the temperature is increased to a defined level, which permits the adsorbed component to be drawn off. In a typical TSA process, two or more adsorption zones and two or more desorption zones are operated in an alternating manner to provide continuous treatment.
Various classes of adsorbents are known to be suitable for use in PSA and TSA systems, the selection of which is dependent upon the gas mixture components and other factors generally known to those skilled in the art. In general, suitable adsorbents include molecular sieves, silica gel, Y-type zeolite, X-type zeolite, activated carbon, and activated alumina.
A key aspect in separating CO2 is the identification of a suitable adsorbent. Although several types of adsorbents may be employed for the adsorption of CO2, important factors for an efficient process include choosing an adsorbent that has strong affinity for CO2 and also has an appropriate sorption capacity as well as desorption capability. Thus, a heretofore unaddressed need exists in the industry for alternative adsorbents that are capable of adsorbing and desorbing CO2.